1. Field of the Invention
The present invention relates to a retrieval system for retrieving a desired frame of a web-like microfilm by transporting it and counting marks applied on the microfilm.
2. Related Background Art
A prior art retrieval system (U.S. Pat. No. 4,665,318) comprises a light source such as a halogen lamp, an optical fiber, a photo-electric converter, an amplifier, a comparator and a counter. A web-like microfilm 100 as shown in FIGS. 5A-5C is transported and illuminated by the light source, opaque flip marks which interrupt a transmitted light from the microfilm 100 are detected by mark detection means, and the detected marks are counted by the count means to retrieve a desired image frame 102.
As shown in FIG. 5A, in order to secure the retrieval of the microfilm 100, a transmitted light intensity (density) of a transparent leading portion of the microfilm 100 is detected to determine a reference prior to the retrieval, and when the image frame 102 is to be retrieved, an output of the photo-electric conversion element which senses the light from the mark is compared with the reference (threshold) to determine the presence or absence of the flip mark, and the detected flip mark is counted by the count means.
The reference is determined based on the detected transmitted light intensity at an area Y spaced from the leading edge 101 of the microfilm 100 by a predetermined distance X. Accordingly, in order to accurately detect the light intensity, it is necessary that the length of the microfilm 100 from the leading edge 101 to the leading flip mark and the transparency of that area are constant.
The prior art system includes the following problems.
1 The leading portion of the microfilm 100 may be bent or broken by a load by a film load/unload mechanism in an auto-loading mode or by a jam. In this case, since that portion is cut away, the leading portion of the microfilm 5B is so shortened that the area Y overlaps with the image frame 102. As a result, the correct detection of the transmitted light intensity is not attained.
2 On the other hand, where the leading portion of the microfilm 100 is short as shown in FIG. 5B, a transparent leader tape 104 may be spliced to the leading portion of the microfilm 100.
However, a base density of the microfilm 100 and a base density of the leader tape 104 may materially differ from each other, or the leader tape 104 may include a dark area. When such a leader tape 104 is illuminated by the light source and the transmitted light intensity is detected to determine the reference, there is a risk that an error is included in the count of the flip marks 103 when the microfilm 100 is retrieved.
3 When the splice tape 105 is black (opaque), it interrupts the illumination light from the light source and it is misdetected as a flip mark. Thus, the count means miscounts the splice tape 105 as the flip mark, and an error results in the retrieval.
The counting of the flip mark is started from the count start point A which is spaced from the leading edge 101 of the microfilm 100 by a distance Z. Accordingly, in order to accurately count, it is necessary that the length of the microfilm 100 from the leading edge 101 to the count start point A is constant.
However, the prior art system involves the following problems.
4 The leading portion of the microfilm 100 may be bent or broken by a load by a load/unload mechanism for the auto-loading or by a jam. In this case, since that portion is cut away, the distance from the leading edge 101 to the leading flip mark 103 is shortened as shown in FIG. 5B. As a result, the leading flip mark 103 is not counted and the retrieval fails.
5 On the other hand, when the microfilm 100 is shortened as shown in FIG. 5B, a leader tape 104 may be joined to the microfilm 100 by a splice tape 105 as shown in FIG. 5C. However, not only the joining work is troublesome but also the splice tape 105 may overlap the count start point A or the splice tape 105 may include a dark area, which results in an error in the count.